More specifically, the invention relates to the production of such material or structures by a casting process that involves infiltrating molten metal around a removable refractory mould or space holder that defines the foam structure. There are already several processing routes for metal foams that fall into this class, reviewed for example in [M F Ashby, A G Evans, N A Fleck, L J Gibson, J W Hutchinson, H N G Wadley “Metal Foams: A Design Guide” Butterworth-Heinemann, Boston, (2000)], [J Banhart, Progress in Materials Science 46 (2001) 559-632], [Y Conde, J-F Despois, R Goodall, A Marmottant, L Salvo, C San Marchi & A Mortensen, Advanced Engineering Materials 8(9) 795-803 (2006)]. Due to the complex interconnected porosity, normally exceeding 40% of the total volume of the article, the requirements of such a mould or space holder and hence methods by which they are made are generally different from those used to shape hollow castings.
A method using investment casting with a polymer precursor is disclosed in [Y Yamada, K Shimojima, Y Sakaguchi, M Mabuchi, N Nakamura, T Asahina, T Mukai, H Kanahashi & K Higashi, Journal of Materials Science Letters, 18 (1999) 1477-1480]; it is also guessed that this is the method used to produce “Duocel metal foams” currently marketed by the ERG Materials and Aerospace Corporation (http://www.ergaerospace.com/), [M F Ashby, A G Evans, N A Fleck, L J Gibson, J W Hutchinson, H N G Wadley “Metal Foams: A Design Guide” Butterworth-Heinemann, Boston, (2000)]. In this method, an open-celled organic foam of, e.g., polyurethane is filled with a refractory slurry, typically an investment casting moulding compound, which is cured after which a heat treatment is used to densify the mould and remove the initial polymer precursor. Metal is cast into the mould so formed, and the mould material will then be removed using conventional methods, e.g. by mechanical shaking or with a water jet.
U.S. Pat. No. 3,052,967 cited by [J Banhart, Progress in Materials Science 46 (2001) 559-632] discloses a method of manufacturing a foam using a preform of sand particles held together with a binder that decomposes at high temperatures, allowing the sand to be shaken out.
If casting is sufficiently rapid, then sintered polymer granulates can be used as the preform with aluminium. After casting, a thermal pyrolysis treatment is used to remove the polymer. This method is, for example, described by the Fraunhofer Institute in Bremen, http://www.ifam.fraunhofer.de/index.php?seite=/2801/leich tbauwerkstoffe/offenporoese-strukturen/&lang=en.
Alternatively, sintering of metal powder around removable space holders may be used. Powder of the desired metal is mixed with a sufficient quantity of particles of a material that can be removed either by water or a suitable heat treatment, before sintering of the powder to produce a cohesive material. During this stage the space holder particles retain the porosity in the foam. Examples of space holders used include salt [Y Y Zhao, D X Sun, Scripta Mater. 44 (2001)] and urea [B Jiang, N Q Zhao C S Shi, J J Li, Scripta Mater. 53 (2005) 781-785] (both removed by dissolution in water).
A relatively simple method uses grains of normal table salt to define the foam porosity, as described in U.S. Pat. No. 3,236,706. If the grains percolate, then after infiltration of the intergranular spaces with molten metal and solidification of the latter the salt may be removed by dissolution in water. Research has developed this process to vary the foam porosity (in the range 0.6-0.9), pore shape (by using different shapes within the set of possible salt crystal forms), and pore size (in the range 5 μm-2 mm), see [C San Marchi & A Mortensen, Acta Materialia 49 3959 (2001); C San Marchi, J-F Despois & A Mortensen, Acta Materialia 52 2895 (2004); J-F Despois, Y Conde, C San Marchi & A Mortensen, Advanced Engineering Materials 6(6) 444 (2004); C Gaillard, J-F Despois, & A Mortensen, Materials Science and Engineering A 374(1-2) 250 (2004); R Goodall, A Marmottant, L Salvo & A Mortensen, Materials Science and Engineering A 465 (1-2) 124 (2007)]. However, the method is limited by the size and shape of available salt crystals, the fact that salt grains larger than about 0.5 mm diameter cannot be compacted in the same way as the smaller grains, and the slow rate of preform removal by dissolution.